For the past 50 years, our efforts to detect extraterrestrial civilizations have largely focused on the search for radio emissions. But this is hardly the only strategy at our disposal. Here are 14 intriguing ways we could prove that aliens really exist.

If we're going to find an extraterrestrial intelligence (ETI), we're going to recognize their signatures. These signs can be organized into three basic types: spectral signatures, physical artifacts and structures, and remnants, or byproducts, of their technological activities. Some of these signatures could be deliberate attempts to get our attention, while others are simply leakage.

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Should we detect these signatures, however, the challenge will in determining which are the work of an ETI and which are the products of some unknown or bizarre natural phenomena (like the time we thought quasars were an alien beacon).

Here's what we should be looking for:

1. Radio Signals

As noted, this is the most well-known of the SETI targets. Radio signals emanating in narrow, focused bands could indicate the presence of intelligent life.

We've been leaking electromagnetic waves of various intensities and frequencies for well over a century, the remnants of TV broadcasts, mobile phone conversations, satellite transmissions, along with military, civil and astronomical radars. It's fair to assume that ETIs are doing the same thing (particularly if intelligence converges around similar developmental trajectories). Radio signals could also be broadcast deliberately (called messages to extraterrestrials, or METI), but that's risky.

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Unfortunately, radio signals decrease in intensity as they leak out into the cosmos. But depending on a signal's strength and frequency, these waves can propagate for cosmologically vast distances and still carry enough information to connote the presence of intelligent life. Astronomer Jacob Haqq-Misra says a transmitter beaming power of 0.8 MW and a frequency of 2,380 MHz could be picked up by a "watcher" with a 1 km2 receiving antenna at distances of up to 200,000 light years. That said, the receiver would have to focus on a specific spot for a really, really long time to pick up a meaningful signal.

2. Optical Signals

For example, aliens could use powerful lasers to communicate across stellar distances at various optical wavelengths. Lasers are particularly effective for point-to-point communication, so we can conceive of automated systems that point lasers at candidate systems. Lasers have tight beams which can carry loads of data. Moreover, unlike visible light (which can be blocked by dust in the galaxy), light in the infrared spectrum can easily penetrate our galaxy's dusty medium.

To date, Berkeley's SETI program has scanned about 20,000 objects (mostly sun-like stars) at 10-minute intervals, so if the laser blinking frequency is longer than that, we're out of luck. The Berkeley researchers are looking for pulsed bright flash signals or continuous signals. Nothing interesting has been discovered so far.

3. Microwave Signatures

The modern search for aliens officially began in 1959 with the publication of a paper by Philip Morrison and Giuseppe Cocconi. Their article, "Searching for Interstellar Communications," proposed a search of nearby sun-like stars for deliberate microwave radio signals on the 21-centimeter hydrogen line.

4. Artificially Appearing X-Ray and Gamma Ray Bursts

ETI's could also get our attention through the deliberate transmission of high-energy x-rays and gamma ray bursts.

But this would require hideous amounts of energy per bit — something that only a super-powerful civilization would be capable of. According to John A. Ball, an advanced ETI should be capable of transmitting a two-millisecond pulse encoding 1x1018 bits of information. That's "comparable to the estimated total information content of Earth's biosystem — genes and memes and including all libraries and computer media."

5. Neutrino Communication

Image: IceCube Collaboration.

High-energy neutrinos could also be used as a communications medium. They're particles with no charge but with a discernable energy and spin. These ghostly particles have no rest mass and can travel cosmological distances at the speed of light. With this in mind, some scientists say we should start sniffing neutrinos for traces of messages (once we have the technology, of course — and we're getting there). Specifically, we should be on the look-out for unusual or concentrated neutrino emissions. As J. G. Learned and S. Pakvasa have noted, "Such signals from an advanced civilization, should they exist, will be eminently detectable in existing neutrino detectors."

6. Gravitational Waves

Image: NASA.

Similarly, an ETI could send gravitational waves our way. They could do this by shaking a planet or something else with extreme mass in manner that could produce an obvious signal. One of the advantages of gravitational waves is that they travel at the speed of light. But it may be some time before we have a sensitive enough wave detector. That said, this seems like a rather laborious and energy-intensive way to send a signal.

7. Industrial Waste Signatures

Image: Suicup via Wikimedia Commons.

Spectrographic analysis of exoplanetary atmospheres could indicate the presence of elevated levels of carbon dioxide (and other industrial bi-products) beyond what's naturally possible. The challenge for us will be in figuring out which elements, and in which ratios, these gases can appear exclusively as the result of an intelligent civilization at work.

8. Calling Cards

French astronomer Luc Arnold says we should be on the hunt for deliberately placed artificial objects.

Using the transit method of detection, we could find oddly shaped objects orbiting around stars — like a perfectly triangular structure or a two-screen object (think of a rectangle with a box-like hole cut in the middle). These so-called calling cards would alert us to the presence of an ETI — though this strategy will only work for those systems whose orbital plane aligns with our line of sight.

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"A remarkable lightcurve would be created by free-flyers transiting their star successively in a distinguishable manner," writes Arnold. "At each period, we would observe a series of transits whose number and timing would claim its artificial nature and will of communication."

We should also look for signs of Dyson Spheres in neighboring galaxies, what would appear as large empty swaths of space. This could indicate the spread of a civilization approaching Kardashev III status.

10. Exoplanets That Appear Out of the Ordinary

There are also supramundane planets to consider, an idea conceived by Paul Birch.

Image: Steve Bowers.

These are planets that have been completely reworked by an advanced civilization and would include Earth-like terrestrial planets or gas giants.

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For example, an ETI could create a new surface above a Jupiter-like planet at hundreds of thousands of kilometers from its center of mass, where the gravity is more like an Earth-like planet. This surface would be suspended using so-called Mass Stream Technology. Orion's Arm explains:

Mass Stream Technology uses mass particle beams which encircle the planet or star to support structures above it; by exerting thrust magnetically against these beams (known as dynamic orbital rings), suborbital structures can be suspended at any height. Dynamic compression members and dynamic orbital rings using mass stream technology would be configured into a framework around the planet which would support platforms, which could in turn support a large biosphere. Individual platforms could then be extended into bands which could later be widened into a complete shell.

Speaking of orbital rings, we should look for those, too — another idea proposed by Birch. These are hypothetical rings in low planetary orbits that rotate above orbital speed and have fixed tethers descending down to the surface.

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Detecting supramundane planets poses a challenge, but inexplicable spectrographic atmospheric signatures would be a good place to start.

Another thing to look out for are free-floating Steppenwolf Planets. Dorian Abbot and Eric Switzer contend that a rogue planet (one ejected from its planetary system) could still foster a liquid ocean under ice as a result of geothermal flux. Such a planet could be detected from solar radiation, while its thermal emissions could be pinpointed in the far infrared if it passes within 1,000 AU of Earth.

11. Artificial Illumination

Less speculatively, we could simply look for artificial lighting on exoplanets (or megastructures, for that matter).

12. Transiting Space Habitats

Image: Steve Bowers.

If a space habitat is large enough, like a Tsiolkovsky or O'Neill habitat, it could be spotted using the transit method (i.e., detecting the slight and temporary dimming of a distant star). Other habitats that could be large enough include Ring Worlds, Halos, and Bishop Rings, though it would have to be a rather massive rotating wheel space station).

14. Probes

Self-replicating spacecraft, or probes, are another distinct possibility. Signs of an ETI could exist in our very own solar system — we just have to figure out where to look. For example, a Bracewell communications probe could be idling nearby, waiting for us to cross some kind of technological threshold.